1. Field of the Invention
This invention relates generally to the field of paving and specifically to a surface profiler used in a paving operation.
2. Description of the Related Art
In laying asphalt pavement roadways and the like, it is widespread practice to employ so-called "floating screed" paving machines. These machines include a tractor-like main frame having an engine for propulsion and for material distributing functions. Typically, there is a material receiving hopper at the front of the paver arranged to receive hot asphalt material from a truck as the paving machine advances along the roadbed. Slat conveyors or the like are provided to convey the material from the hopper, at the front of the machine, toward the floating screed, at the back of the machine. Immediately in front of the screed, there is typically provided a distributing auger, which receives the raw asphalt material from the slat conveyor and conveys it laterally so as to distribute the material along the front edge of the screed. As the machine advances along the prepared roadbed, the raw asphalt material flows under the screed, which levels, smooths and compacts the material to provide a continuous, level pavement mat.
In a typical floating screed asphalt paver, the screed is attached to a pair of forwardly extending tow arms that engage the paver frame at their forward extremities. These tow arms are also connected to the paver frame by hydraulic or other actuators arranged to adjust the vertical position of the tow arm extremities in relation to the paver frame. By effecting proper control over the position of the tow arm forward extremities, the screed is maintained in relation to a reference plane or a reference element substantially independent of the irregular vertical motions of the paver frame itself. Thus, it is possible to cause the floating screed to lay a pavement mat which is smooth and level.
Effective control of the screed may be achieved by means of a suitable position sensing device, for example, which is carried by one or both of the tow arms or other forward projections of the screed and arranged for contact with a predetermined reference surface. When the position sensing device becomes either higher or lower than is indicated by the reference surface, as with changing loads upon paver frame and/or irregularities in the roadbed surface, the tow point is caused to be controllably raised or lowered relative to the paver frame to maintain a constant relationship between the position sensing device, called the grade sensor, and the reference. In many applications, grade control is provided at only one side of the machine. For controlling the other side, a slope control can be provided, which functions to maintain a constant relationship between screed ends at opposite sides, either on a level basis or with a predetermined transverse slope.
In conjunction with this type of tow point control, it is important to provide an appropriate reference for the position sensing device. One type of reference is a stringline or wire suspended beside the surface to be paved. The sensor on the paver tow arm engages and follows the stringline to maintain the tow arm at a desired distance from the reference, thereby controlling levelling of the mat by the screed. U.S. Pat. No. 3,604,512 to Carter shows an example of a grader using a stringline or wire reference. The stringline is time-consuming to install and obstructs access to the roadway.
The base surface and fresh mat can be used as a reference with a mobile reference beam that is carried along with the paver as it moves over the roadway base surface. An arrangement of this type is described and claimed in U.S. Pat. No. 3,259,034 to Davin, incorporated herein by reference. In the arrangement of the Davin patent, an elongated beam structure is provided with a plurality of independent supports. The individual supports follow minor deviations in base contour without significantly affecting the position of the reference beam as a whole, and the mobile reference beam thus provides a suitably accurate, averaged reference plane representing the grade to which the pavement mat is to be applied. A sensing device carried by forward projections of the screed engages the reference beam near its center, to enable the screed to be maintained in a predetermined relationship to the moving reference beam. U.S. Pat. No. 3,846,035 to Davin, incorporated herein by reference, discloses a moving reference beam arrangement in connection with the laying of wide pavement mats, utilizing a combination of reference beams, one being towed ahead of the screed and auger, supported on the roadway base grade, and the other being towed behind the screed and auger, supported on the just-laid asphalt mat.
Another type of system that uses the base surface as a reference is available from Paveset America, Inc. A survey of the paving site is prepared prior to paving and survey data is input to a computer. A beam with shoes or skis senses the base surface and a wheel measures forward travel. The computer controls the screed based on the survey data and forward travel distance.
In some cases, a laser is used as a reference. A laser emitter is set at a certain position near the paving site. A laser sensor determines the elevation of the screed or other levelling device with respect to the laser. The laser emitter is set up after a survey of the site and must be moved periodically as the paver progresses along the roadway. Laser references are shown in U.S. Pat. No. 5,288,166 to Allen, U.S. Pat. No. 5,288,167 to Gaffard, U.S. Pat. No. 5,328,295 to Allen. U.S. Pat. No. 5,333,966 to St-Louis uses a laser to sense distance from a reference during a road painting operation.
After a pavement is laid by a machine such as those described above, one of the most important measures of the quality of the newly paved road surface is smoothness, that is, the number and size of bumps and dips in the pavement. Smooth roads require less maintenance and help conserve fuel. They also provide for a more comfortable ride. Because of the importance of smooth roads, most contractors must adhere to strict specifications concerning the smoothness of the roads they construct. A road which does not meet the specifications may result in the forfeiture of part of the contract price or may require grinding or filling parts of the pavement, both of which are costly to the contractor. On the other hand, pavement which exceeds specifications for smoothness may result in bonus payments to the contractor. Thus, it is desirable to obtain smoothness data on a newly paved road to determine whether specifications are being met.
A number of devices have been used for measuring the smoothness or "profile" of a road. One profiler currently in use is the profilograph, which is an elongated beam or frame supported on several wheels. The beam establishes a datum from which deviations in the road surface can be measured. A sensing wheel rolls on the surface and moves vertically as it travel over bumps and dips in the road. Originally, profilographs were entirely mechanical devices which used a linkage to transmit the vertical movement of the sensing wheel to a pen which traced a plot of the road surface on a moving roll of paper. The profiler plots the elevation of the surface as a function of distance travelled. Typically, a calibrated wheel is used to measure the distance. The plot is analyzed by laying a template with a "blanking band" over the plot. The blanking band defines a tolerance and blanks out minor aberrations.
Profilographs have advanced to the point where data from the sensing wheel is transmitted electrically and can be printed or stored in a computer for later analysis. Some computers provide the capability to automatically analyze the plot by applying an electronic blanking band. It is desirable to obtain profile information soon after laying a fresh mat. Profilers mounted with paving apparatus are shown in U.S. Pat. No. 3,675,545 to Anderson and U.S. Pat. No. 5,362,177 to Bowhall, incorporated herein by reference.
The Global Positioning System (GPS) is a satellite navigation system that includes a plurality of satellites stationed in geosynchronous orbit. These satellites receive signals from fixed ground stations and transmit signals that can be used to determine the position of a receiver adapted to process the signals. GPS provides two positioning services: precise positioning service (PPS), which is reserved for military use and standard positioning service (SPS), which is available to the public. The satellites are synchronized to an atomic clock. A receiver synchronized with an atomic clock can measure the propagation time of signals, and therefore the distance, from three satellites. A user can then determine the position of the receiver in three dimensions. Where the receiver is not synchronized to an atomic clock, measuring an apparent propagation time from a fourth satellite permits correction of any error in the receiver's clock. If positioning in only two dimensions is required, signals from only three satellites are necessary. The receiver must also account for Doppler frequency shifting of the signal resulting from motion of the satellite and motion of the receiver. The signals can also be used to determine time of day and velocity of the receiver. Receivers that will provide position information based on GPS signals are commercially available.